The invention generally relates to fuel cell stack rejuvenation.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide a larger amount of power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair are often assembled together in an arrangement called a membrane electrode assembly (MEA).
Over the lifetime of the fuel cell stack, the performance of the stack may gradually decrease due to the accumulation of water in the stack. In this manner, water may accumulate in flow channels in the cathode and anode regions of a particular fuel cell. This accumulated water at least partially blocks the flows channels and interfere with the flow of reactant gas between the flow channels and the MEA.
A possible way to remove the accumulated water is to disassemble the flow plates of the fuel cell stack to gain access to both sides of each fuel cell. However, such disassembly may consume a significant amount of time. Another way to remove accumulated water may be to blow gas (air or nitrogen, as examples) into a manifold inlet opening of the stack to create a differential pressure that causes the water to blow out of the corresponding manifold outlet manifold opening. However a problem with this technique is that the gas diffuses through the various orifices of the stack, thereby requiring high pressurization of the gas.
Thus, there is a continuing need for an arrangement and/or technique that addresses one or more of the problems that are stated above.
In an embodiment of the invention, a technique includes applying a vacuum to a manifold of a fuel cell stack to remove accumulated water from the stack. As an example, a fuel cell system may include a fuel cell stack that is capable of accumulating water. A vacuum system of the fuel cell system applies a vacuum to the fuel cell stack to remove at least some of the accumulated water.